Preheated air is conventionally injected into a shaft furnace by means of an assembly, known as the tuyere stock, which provides a connection between the hot air bustle pipe and a tuyere, installed in the furnace wall, which opens into the furnace shaft. Typically, as shown in FIG. 1, the tuyere stock 10 is made up of a number of separate elements each comprising an external metal sheath and an internal refractory lining.
The tuyere stock 10 comprises                a downpipe 11 extending obliquely downwards from a hot air bustle pipe 1,        an elbow 12        a blast pipe 3, having a substantially horizontal axis, attached rigidly to the elbow, the front end of the blast pipe being held in contact with the rear end of the tuyere 4.        
The copper tuyere 4, cooled by internal water circulation, is fitted in a tuyere holder, or tuyere cooler, 42 attached to the metal wall 21 of the blast furnace and extending through the internal refractory lining 22 of this wall.
The blast pipe 3 comprises an external wall 31 of steel, internally lined with a refractory material 32. As shown in FIGS. 2 and 3, the external wall extends towards the front end, or tip, 33 of the blast pipe to form spherically shaped wall resting swivellably against a frustoconical wall 41 formed on the rear end of the tuyere 4.
The tuyere stock is supported on the external metal wall 21 of the furnace by articulated tie rods, such as the upper tie rod 23 and lateral tie rods 24, the latter holding the blast pipe against the tuyere. These tie rods provide the tuyere stock assembly with a certain freedom of movement in order to absorb the thermal deformations, both of the furnace wall and of the various elements of the tuyere stock, which inevitably arise from fluctuations in temperature.
In order to provide a seal between the various elements despite the above-mentioned deformations, it is known to use bellows compensating systems 13 which make it possible to absorb relative motion, whether involving axial displacement or pivoting, at the connection between two elements, while still maintaining a seal, the tie rods providing a mechanical connection between the elements joined by the bellows compensators. Such compensator systems are described for example in EP 0 453 739, and are typically used to provide a seal between the bustle pipe and tuyere stock and may also be used between the air downpipe and the elbow to which blast pipe is attached.
Such compensator systems are all the more useful for providing the tuyere stock a certain ability to deform given that it is furthermore necessary to provide the best possible seal at the joint between the tip of the blast pipe and the tuyere, just where such systems cannot be used in particular due to the configuration in the environment of the joint, located in the thickness of the refractory wall of the furnace. This joint is thus provided simply by the tip of the blast pipe resting in centered manner against the rear end of the tuyere solely under the action of the forces arising from the weight of the tuyere stock and the tension exerted by the articulated tie rods 24 connecting the tuyere stock to the external metal wall of the furnace. In order to permit thermal deformation which might bring about an angular offset between the blast pipe and the tuyere, the blast pipe tip has a convex spherical surface in contact with a corresponding frustoconical or convex surface formed on the rear end of the tuyere. The spherical surface of the tip of the blast pipe is formed by the end of the external steel wall of the blast pipe. The tuyere is conventionally made of copper.
This arrangement, forming a ball joint connection between the blast pipe and tuyere, makes it possible to ensure centering of the blast pipe on the tuyere simply by the former resting against the latter and the metal-on-metal contact between the steel of the blast pipe and the copper of the tuyere makes it possible, during relative angular displacement thereof, to maintain a substantially linear contact between them and therefore a certain level of seal between blast pipe and tuyere.
U.S. Pat. No. 3,545,736 also discloses a system wherein an annular space is provided between a tubular metal jacket and the refractory part of the blast pipe. The jacket comprises a first end placed against the tuyere and capable of pivoting thereagainst, and is pressed against the tuyere by the elbow of the tuyere stock which itself rests in sealed manner against the second end of the jacket. The object of this arrangement is to limit the temperature difference between the outside and inside of the refractory part of the blast pipe by allowing hot gases to get into the annular space via an axial spacing maintained between the elbow of the tuyere stock and the blast pipe. In this system too, a certain degree of sealing may be obtained between the first end of the jacket and the tuyere when they undergo relative angular displacement.
However, just as when the refractory blast pipe is pressed directly against the tuyere, it is not possible for a perfect seal to be provided. Consequently, limited leaks of blast air are currently accepted insofar as said hot blast air is only oxygen-enriched air. However, when implementing more recent flue gas recirculation techniques, this gas is reinjected with the hot blast air. This flue gas is in fact very hazardous and even minimal levels of such leaks consequently cannot be permitted. Even in conventional blast furnaces, there is nowadays a desire to reduce hot blast air leaks for reasons of better safety.